As already known from the state of the art, in all applications the gas arrives to the burner after passing through devices (valves, nozzles and so on), all having in common, from the functional point of view, a calibrated orifice.
With a calibrated orifice having the same size, the gases having a high Wi can supply more thermal energy; the opposite is true for gases having a low Wi.
Each gas is further characterized by a higher or lower propensity to correct combustion.
There are gases having more difficulties in reaching a perfect and complete combustion with a higher emission of pollutants CO and CO2; they are the so called “incomplete combustion limit gases”.
They are always characterized by the highest Wi of their category.
Moreover, there are gases having a higher flame propagation rate and therefore a higher propensity to backfire inside the burner. They are the so called “backfire gases”.
These gases are characterized by having a high Wi which is however lower than the highest of their category.
Finally, there are gases having a lower flame propagation rate and therefore a higher propensity to a flame detachment from the burner; they are the so called “flame detachment gases”.
These gases are characterized by the lowest Wi of their category.
In order to facilitate the correct matching between gases distributed in the market and gas appliances, in Europe gases have been classified according to homogeneous groups, as well as according to families (as previously mentioned).
Within the natural gas family, in fact, groups H, L and E have been identified.
Group H comprises gases having a Wi comprised between 41.01 and 49.6 MJ/m3 and has G20 methane as reference gas.
Group L comprises gases having a Wi comprised between 35.17 and 40.52 MJ/m3 and has G25 as reference gas.
Group E comprises gases having a Wi comprised between 36.82 and 49.6 MJ/m3 and has G20 methane as reference gas.
Within the liquid gas family (commonly referred to as GPL) groups B and P have been identified.
Group B comprises gases having a Wi comprised between 68.14 and 80.58 MJ/m3 and has G30 butane as reference gas.
Group P comprises gases having a Wi comprised between 68.14 and 70.69 MJ/m3 and has G31 propane as reference gas.
By reading the various Wi related to the first gas family, namely the most widespread, it is clear that group E contains gases with the broadest Wi spectrum.
As a consequence, it is decidedly more complex to manufacture products provided with burners suitable to this gas group, or which are indifferently suitable to gases of the groups H and L.
On the other hand, products which are suitable to this kind of gases are the most valued, because they can be interchangeably installed almost all over Europe without limitation.
Unfortunately, in order to allow the burners to work correctly and without flame detachment with “limit” gases, having a lower Wi, the burners must work with reference gases having a particularly low air/gas ratio, namely with particularly gas-rich mixtures, all at the expense of combustion hygiene.
This explains the constant search for solutions designed for making burners work with gases having the broadest Wi difference.
From the technical point of view, the way to get an excellent combustion has long been known.
In fact, it is well known in the art that to get an excellent combustion it must take place with a mixture having an amount of excess air comprised between 30% and 35%.
As it is also known, the air/gas ratio in a fuel mixture is synthetically indicated with the parameter λ.
It represents the ratio between the amount of air used in the combustion process and the amount of air stoichiometrically required.
For methane combustion, for instance, the amount of air stoichiometrically required corresponds to 9.52 m3 for each m3 of methane, corresponding to λ=1.
Actually, if the combustion took place in the presence of the stoichiometrical amount of air only, it would have a very high production of unburned by-products, and in particular of CO.
Therefore, combustion always takes place in the presence of an amount of excess air, then with λ>1.
For premix burners such optimal amount of excess air, as already stated, has been experimentally identified in a value comprised between 30% and 35%; namely a λ comprised between 1.30 and 1.35.
It has been experimentally confirmed that such optimal value is suitable to any kind of gas belonging to the different groups of the two available families; therefore, besides being suitable to reference gases, it is also suitable to limit gases.
However, traditional air/gas systems cannot distinguish the kind of gas they are supplied with; moreover, if they worked with the optimal λ value corresponding to 1.33, by inserting limit gases having a lower Wi they would constantly risk to get blocked because of a flame detachment.
As a consequence, a traditional air/gas system is operated with reference gas G20 at λ values corresponding to 1.25.
By inserting the incomplete combustion limit gas G21, the λ value becomes 1.17, with a subsequent high emission of CO and NOx; by inserting the flame detachment limit gas G231, the λ value becomes 1.50 with a subsequent risk of flame detachments and subsequent burner block thanks to the safety device.
This long introduction serves to understand the protracted efforts made to find solutions which would allow the burners to work with a constant air/gas ratio regardless of the kind of gas.
In order to improve the understanding of the present invention, reference is made to an embodiment of an apparatus for adjusting and controlling the combustion according to the prior art; said known embodiment is showed in FIG. 1.
The apparatus 100 of FIG. 1 belonging to the prior art comprises:                a Venturi mixer 15, placed in a mixing pipe 10, in which a fuel gas supply duct 22 flows; therefore in this case the mixing area (ZM) is in correspondence of the Venturi mixer 15; and        a pneumatic gas valve 20 (fed by gas through a supply duct 21), feeding gas in an amount corresponding to the depression generated downstream of the valve by the Venturi mixer 15 and, therefore, corresponding to the amount of air passing through it.        
The gas valve 20, the so called “pneumatic valve”, is a device providing for both adjustment and safety.
It can be schematically showed as a device inside which two shutters are provided.
The first shutter performs the safety function, whereas the second shutter provides for the adjustment of the gas flow.
The first shutter is connected to the safety system arranged in an electronic control unit (CE) and based on the detection of the presence of the flame.
The second shutter is connected to the adjustment system operated by the depression generated by the Venturi mixer 15.
The apparatus 100 further comprises:                a fan 30 whose impeller is housed in the mixing pipe 10 and is arranged downstream of the gas/air mixing area (ZM);        a burner 40, arranged downstream of the fan 30, preferably being of the perforated duct kind; in other words, the burner 40 looks like a metal pipe closed at the bottom and provided with a plurality of through holes from which the air/gas mixture comes out, said mixture being ignited, in a known way, by an electric device (not shown). It is thus created a flame (FLM), substantially evenly distributed over the entire outer cylindrical surface of the burner 40; and        a safety system based on the flame (FLM) detection and developed by means of a safety spark plug 50 whose electrode 51 is under tension with respect to the metal mass of the burner 40; and it is known that in the presence of the flame (FLM) there is the passage of an electric current (very small and rectified, since the flame acts as a rectifier of alternating current) between the electrode 51 and the metal mass of the burner 40; this current is detected by means of known systems by the electronic control unit (CE) which, at the same time, generates the voltage difference required for the passage of the current.        
As also shown in FIG. 1, the electronic control unit (CE) is electrically connected to the gas valve 20, to the fan 30 and to the safety spark plug 50.
Said apparatus 100 is characterized by the following features:                the fan 30 determines the air flow required for the perfect and complete combustion of the gas;        the gas valve 20, operated by the Venturi mixer 15, supplies gas in an amount proportional to the air flow;        the electronic control unit (CE) constantly verifies the presence of the flame (FLM) on the burner by means of the spark plug 50.        
However, the apparatus 100 of the prior art still has the aforesaid drawback related to the inability to adapt to different kinds of gas.
By varying the Wobbe index of the incoming gas, the air/gas ratio values in the burner significantly vary too, with a negative impact on the emission of pollutants CO and NOx and sometimes with problems of flame detachment due to excess air and a consequent block of the gas flow through the gas valve.